(a) Field of the Disclosure
This invention relates to a method of arc welding in a chamber in which there is a gaseous environment. The method is particularly intended for operation under water and at superatmospheric pressure.
(B) Description of the Prior Art
With the growth of the offshore industry it has become essential to be able to make welds under water to a standard that satisfies certain nationally and internationally recognised welding codes. Attempts have been made to weld underwater without taking any steps to protect the welding arc from water. These have been unsuccessful, partly because water entering the arc becomes dissociated and the hydrogen thus formed is dissolved in the weld pool. The rapid quenching effect of the water on the weldments creates hard martensitic structures in the heat affected zone (HAZ) which are susceptible to hydrogen induced cracking, owing to hydrogen diffusion into the heat affected zone, particularly in joints subject to restraint.
It has been proposed to prevent water entering the arc by using a welding torch with a nozzle which is adapted to form a protective annular curtain of water spaced apart from the welding arc. With this piece of equipment it has been proposed to use a semi-automatic welding method with a consumable welding wire and a shielding gas including a large proportion of argon or carbon dioxide. This method is described inter alia in the Journal of the Japan Welding Society, 1974, pp 23 to 30, and pp 141 to 146.
However, when it is required to vary the angle of the welding torch in relation to the work, (ie in "out-of-position" welding) as in manually operated GMA welding, the water curtain shielding cannot be fully maintained to prevent relatively large quantities of water from entering the arc and the resultant turbulence seriously impairs the welder's visibility. Even when a constant angle is maintained so as to preserve the water curtain (eg in automatic straight line welding) fume emitted by the consumable welding wire will tend to be confined to the region of the arc by the water curtain thereby also seriously impairing the welder's visibility of the arc.
Alternatively it has been proposed to use `in the wet` a flux-cored wire or a solid wire with a shielding gas under water, without any water curtain. The use of solid wire with a shielding gas is the conventional, so-called GMA welding process. Such proposals have been made in a number of papers, for example, by M L Levin in Marine Technology, Vol. 4, No. 3 June 1973. pp 73 to 77, and by I. M. Savitch in the proceedings, International Conference, Welding in Offshore Constructions, 26 to 28 February 1974, paper no. 20, pp 217 to 220. In the latter case no western investigator has been able to repeat the claimed results. Satisfactory welds may be achieved under water using semi-automatic GMAW by forming under water a fixed chamber, displacing water from the chamber by passing gas into it, and then welding by means of the conventional GMA process in the short-circuiting mode which employs a solid wire. Such a process is described in U.S. Pat. No. 3,876,852 (Topham). M. L. Levin in `Welding in the Sea`, Marine Technology, Vol. 4 No. 3 June 1973 pp 73-77 also refers to using a semi-automatic MIG welding process in a gas filled chamber under water in order to make `top quality` welds. He specifically states that the welding electrode should not have any flux. P. L. J. Leder and F. W. Lunau in a paper entitled `High duty welding` published in the Australian Welding Journal 18, No. 5 pp. 149-159, September-October 1974 also refers to GMA welding under water, this time in a small hand-held gas-filled chamber having transparent walls.
We have surprisingly found that despite the aforementioned publications to use underwater a semi-automatic GMA (ie solid wire) arc welding process in a chamber containing gas this process often fails to produce sound welds, there often being lack of fusion between the weld metal and the parent metals ie the weld metal solidifies before adequate fusion with the parent metal has taken place.